Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
646 PM EDT Fri Apr 03 2020
Valid 12Z Tue Apr 07 2020 - 12Z Sat Apr 11 2020
...Guidance/Predictability Assessment and Weather/Threats
Highlights...
One feature with ongoing guidance uncertainty is a potentially
quite concentrated storm system that may track into the Gulf of
Alaska by around day 5 Wed. This system may be accompanied by
strong winds along with a period of heavy precipitation focused
most likely over the Panhandle but possibly extending back to the
southeastern coast of the mainland depending on low track.
Dynamic support aloft comes from shortwave energy forecast to be
over the eastern Aleutians/Alaska Peninsula on Tue. GFS/ECMWF
runs through the 12Z cycle have been fairly consistent in
principle. Weaker ensemble means had been similar before the 12Z
cycle. The 12Z GEFS mean came in with a slower shortwave and in
fact phased with the mid-latitude upper low to the south in a way
not seen in other guidance, so that solution appears to be a
low-probability one at this time. CMC/UKMET and other models vary
widely. Preference is with a 12Z ECMWF/GFS blend tilted somewhat
more toward the ECMWF. This provides reasonable continuity for
overall evolution along with a deeper trend from yesterday.
Behind this system, guidance evaluation focuses on the
strength/progression of upper ridging that builds into the North
Pacific/Bering Sea and then possibly the mainland--and in turn
eastward/northward extent of low pressure reaching the western
Aleutians by Wed-Thu and the system initially over the
mid-latitude Pacific. Latest GFS runs are among the most
progressive for the overall pattern from the Aleutians through the
Bering/Sea and mainland. While the 12Z GEFS mean is suspect
farther east, it does provide support for the idea that the
western Aleutians system should stall rather than continue onward
per the GFS. Meanwhile operational ECMWF runs stray a bit to
opposite sides of the spectrum by the latter half of the period,
the old 00Z run being strong/slow/closed with upper ridging and
suppressed with the overall surface pattern versus the new 12Z run
that lets the upper ridge give way enough for the mid-latitude
Pacific system to reach northward into the Aleutians by late next
week. In fact 06Z/12Z GFS also varied greatly for the
mid-latitude low but in opposite sequence of ECMWF runs. The
guidance spread/variability led to the ECMWF/GFS blend used for
the first couple days of the period being transitioned toward a
solution comprising half 00Z ECMWF mean and the rest a split
between the last two ECMWF runs for days 7-8 Fri-Sat. The new 12Z
ECMWF mean seems to provide support for an intermediate solution,
perhaps a bit more upper ridge progression than derived from the
forecast blend but still a fair degree of suppression at the
surface.
Multiple surface highs tracking over the mainland will bring below
normal temperatures for a majority of the period. The North Slope
region should be the one exception, tending to see above normal
readings most days. Uncertain pattern progression late in the
period will determine how much moderation may eventually occur
over western areas. The western Aleutians should see some
precipitation with the system tracking into the area. How much
moisture reaches the rest of the Aleutians and Peninsula will be
sensitive to the uncertain aspects of flow aloft by the latter
half of the week.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Tue-Thu, Apr 7-Apr 9.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Tue-Wed, Apr 7-Apr 8.
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html